A variety of thin-film photovoltaic devices, such as cells and modules, are known. Multilayered transparent conductive oxide (TCO) stacks are commonly used as support and contact (electrode) for these photovoltaic devices. The TCO stacks have opto-electronic properties, which can directly affect the devices' performance. The opto-electronic properties include electrical conductivity and optical transparency, both of which depend on thicknesses of the layers in the multilayered TCO stacks. Optical transparency is related to optical constants such as refractive index (n), and extinction coefficient (k). The refractive index of a material describes how light propagates through that material. It is a measure of how a wavelength and velocity of light are reduced when propagated through the material as compared to the wavelength and velocity of propagation through a vacuum. The extinction coefficient is a constant that relates to the absorption of light in a material.
Thus, to ensure a consistent performance across devices or batches of devices, it is necessary that the thickness and optical constant values of each layer of a TCO stack be consistent from one TCO stack to another. To do so, the thickness and optical constant values of each of the individual layers in the TCO stacks have to be controlled as the stacks are being formed. To control the thicknesses of the layers as the TCO stacks are being formed, the thickness of the layers must be measured in real-time (i.e., as the layers are being formed) or near real-time.
One method that has been used previously to obtain thicknesses of individual layers of a device is to take microscopic cross-sectional measurements of the device. Microscopic cross-sectional measurements includes cutting open a previously-fabricated device to expose its cross-sectional layers and using an electron microscope (or other microscopic measurement tool) to measure the thickness of each exposed layer.
To ensure that the thicknesses of the layers do not deviate too much from one device to another, the microscopic cross-sectional measurements should be taken on a fairly regular basis. Doing so, however, may slow down the manufacturing line as layer deposition may have to be stopped awaiting the results of the measurements to determine whether adjustments to the deposition equipment are needed. Further, since the device is destroyed in order to take the measurements, additional costs may be added to the photovoltaic devices. Obtaining values for the optical constants of the layers is equally, if not more, challenging.
Hence, in-line measurements of the thickness and optical constant values of the layers of a TCO stack would be preferable, not only to prevent the need for device destruction, but also to allow measurements of each layer of the device in real-time. Real-time layer measurements may allow, for example, for real-time correction of undesired manufacturing variances. It would therefore be desirable to perform an accurate real-time, non-destructive calculation of layer thicknesses and optical properties thereof for the layers of the TCO stack.